Mountain Building in Scotland

Transcription

1 c Science: a level 3 course Mountain Building in Scotland Prepared for the Course Team by Kevin Jones and Stephen Blake Block 3

2 The SXR339 Course Team Chair Stephen Blake Course Manager Jessica Bartlett (Course Manager) Other members of the Course Team Mandy Anton (Graphic Designer) Dave McGarvie (Author) Gerry Bearman (Editor) Steve Best (Graphic Artist) Nigel Harris (Author) Kevin Jones (Consultant Author) Jann Matela (Word Processing) Ian Parkinson (Reader) Val Russell (Consultant Editor) Professor Rob Strachan, Oxford Brookes University (Course Assessor) Andy Sutton (Software Designer) The Course Team gratefully acknowledges the contributions of Andrew Bell and Fiona McGibbon who commented on the proofs of this book. Other contributors to SXR339 are acknowledged in specific credits. Front cover: Gearr Aonach (left) and Aonach Dubh (right) from the Meeting of the Three Waters, Glen Coe. (David W. Wright, Open University) Back cover: The view westwards from Strath Fionan, near Schiehallion, central Perthshire, to Loch Rannoch. (Nigel Harris, Open University) The Open University, Walton Hall, Milton Keynes MK7 6AA. First published Copyright 2003 The Open University. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, transmitted or utilized in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without either the prior written permission of the publishers or a licence permitting restricted copying issued by the Copyright Licensing Agency Ltd. Details of such licences (for reprographic reproduction) may be obtained from the Copyright Licensing Agency Ltd. of 90 Tottenham Court Road, London W1P 0LP. Edited, designed and typeset by The Open University. Printed in the United Kingdom by Bath Press, Glasgow. ISBN This book forms part of an Open University course, SXR339 Ancient Mountains: Practical Geology in Scotland. Details of this and other Open University courses can be obtained from the Course Reservations and Sales Office, PO Box 724, The Open University, Milton Keynes MK7 6ZS, United Kingdom: tel. (00 44) For availability of this or other course components, contact Open University Worldwide Ltd, Walton Hall, Milton Keynes MK7 6AA, United Kingdom: tel. (00 44) , fax (00 44) , ouwenq@open.ac.uk Alternatively, much useful course information can be obtained from the Open University s website, 1.1

3 1 Introduction Setting the scene Recognizing ancient mountains Orogeny through geological time Geological time: a brief note Disentangling the continents The collage of ancient orogenic belts in the North Atlantic region What caused the Caledonian Orogeny? The tectonic map of Britain and Ireland The scope of this book 14 2 Britain s oldest rocks: remnants of Archaean crust Introduction The Lewisian Complex The nature, age and origin of the gneiss protoliths Deformation and high-grade metamorphism Basement inliers in the Moine Supergroup Summary of Section Orogenies in the Proterozoic Introduction Palaeoproterozoic rifting, sedimentation and magmatism The Palaeoproterozoic Laxfordian Orogeny Assembly of the Lewisian Complex Formation of Proterozoic crust Synthesis: the broader view of Palaeoproterozoic events Mesoproterozoic events Mesoproterozoic rifting: deposition of the Stoer Group The Grenville Orogeny Neoproterozoic events Early Neoproterozoic continental sedimentation: the Sleat and Torridon Groups Early Neoproterozoic marine sedimentation: the Moine Supergroup The Dava and Glen Banchor Successions Extension-related magmatism Evidence for a Neoproterozoic (Knoydartian) Orogeny? Summary of Section Continental break up and opening of the Iapetus Ocean Introduction The Dalradian Supergroup Dalradian sedimentary basins: seeing through metamorphism Primary rock types and terminology Lithostratigraphic subdivisions of the Dalradian Supergroup Dalradian sedimentation and tectonics Dalradian basin evolution Age of the Dalradian Supergroup Cambrian Ordovician shelf sedimentation in north-west Scotland Summary of Section Arc continent collision: the Grampian phase of the Caledonian Orogeny Introduction Ocean closure during the Grampian phase Ophiolites and the evidence for obduction: a Grampian suture zone? 48

4 4 Mountain Building in Scotland Evidence for an ancient magmatic arc Evidence for an ancient subduction zone: the Clew Bay Complex Summary of Section Structural development of the Grampian Highlands Introduction Structure of the Grampian Highlands The deformation sequence Summary of Section Metamorphism in the Grampian Highlands Introduction The metamorphic map of the Dalradian The relative timing of porphyroblast growth and deformation The absolute timing of metamorphic mineral growth The causes of Grampian metamorphism A synthesis of Grampian metamorphism Summary of Section Summary of Section Exhumation of the Grampian mountains Introduction Uplift and cooling history of the Grampian mountains Magmatism during exhumation The record of exhumation in sedimentary basins Synthesis: time constraints on the Grampian phase How high were the Grampian mountains? Summary of Section Sedimentation and tectonics at a mid-ordovician to Silurian active margin Introduction Mid-Ordovician to Silurian sedimentation in the Midland Valley Terrane Ordovician sedimentation Silurian sedimentation Summary of Section Sedimentation and tectonics in the Southern Uplands Terrane Sedimentation The Southern Uplands as an accretionary prism? Summary of Section Interpretation: regional tectonic framework for the Midland Valley and Southern Uplands Introduction What caused a subduction zone reversal? A missing fore-arc basin? Summary of Section Multiple plate collisions and the end of the Iapetus Ocean Introduction Palaeocontinental reconstructions The global view A model for the closure of the Iapetus Ocean Summary of Section

5 Introduction Tectonics of the Northern Highlands Structure and metamorphism of the Northern Highlands Magmatism and the timing of deformation Regional implications Summary of Section Silurian Devonian strike slip displacements on the Laurentian margin Geometry and amount of displacement Time constraints on fault zone displacements Sinistral displacements in the Southern Uplands Summary of Section Collision of Eastern Avalonia with the Laurentian margin The sedimentary record of a far-felt collision Diachronous collision and terrane linkage Sediment dispersal and basin shallowing Where is the Iapetus Suture? Summary of Section Late Silurian to Early Devonian deformation of Eastern Avalonia The cleavage pattern Time constraints on cleavage formation and deformation Summary of Section Granite magmatism and convergence Origin of the Newer Granites Summary of Section Summary of Section Sedimentation at the end of the Caledonian Orogeny Introduction The Old Red Sandstone and the Devonian Period Distribution and stratigraphy of the Late Silurian to Devonian Basins Sedimentation and tectonics in the Midland Valley Structure of the Midland Valley in the Devonian Period Late Silurian Early Devonian sedimentation A Mid-Devonian hiatus Renewed deposition in the Late Devonian Summary of Section Sedimentation and tectonics in the external basins A southward-migrating mountain front A short-lived Mid-Devonian ocean basin? Summary of Section Orogen-scale drainage and sediment dispersal patterns Summary of Section Legacy 113 Acknowledgements 114 Index 115

6 6 Mountain Building in Scotland 1 Introduction 1.1 Setting the scene Some of Britain s most dramatic scenery is to be found in the Scottish Highlands. The sight of mighty Ben Nevis, the desolate plateau of the Cairngorms, or the imposing landscapes of Glen Coe (pictured on the front cover) can unleash the call of the wild in all of us. Although these landforms were largely carved by glacial activity that ended some years ago, the rocks themselves tell of a much older history. The Scottish Highlands, defined as lying north of the Highland Boundary Fault (Figure 1.1), are primarily composed of metamorphosed sedimentary and igneous rocks intruded by somewhat younger igneous bodies and cut by faults. The metamorphic rocks and igneous intrusions exposed at the surface must have formed within the Earth s crust, and this simple fact indicates that the Highlands are merely eroded stumps of a much higher range of ancient mountains. This book is an account of the origin and demise of that ancient mountain range, based on the geological evidence laid before us in rock exposures. For well over a century, geologists have braved the rain, sun and midges to study the Highlands. Indeed, it was nineteenth-century field geologists working in northern Scotland who established several of the fundamental geological principles that are today almost taken for granted. Notable Victorian scientists such as Ben Peach, John Horne, George Barrow, Edward Bailey, Archibald Geikie, Charles Lapworth and Roderick Murchison were among those who made their names by observing and debating the significance of the rocks of the Highlands. For example, Peach and Horne, with others from the Geological Survey, mapped the rocks of north-west Scotland, recognizing an enormous low-angle fault in which ancient metamorphic rocks had been thrust tens of kilometres westwards over younger sedimentary rocks. This fault is now familiar to geologists across the world as the Moine Thrust (Figure 1.1). Working in the south-eastern Highlands around Glen Clova (Figure 1.1), George Barrow founded the concepts of metamorphic zones and index minerals, which are now universally used to map belts of regional metamorphic rocks. Another example, albeit from further south, is James Hutton s recognition, in 1788, of an unconformity at Siccar Point (Figure 1.1), where red sandstones (of Devonian age) had been deposited on upturned older (Silurian) strata. This and similar observations nearby led him to the revelation that sedimentary deposition had been interrupted by significant earth movements and erosion, requiring a truly dynamic Earth and eons of time. Now, in the era of sophisticated geochemical, geochronological and geophysical methods, geologists are still working to make further sense of the region s dramatic geology in terms of plate tectonics and continental drift. So, the prodigious terrestrial displacements, recognized by Geikie to have formed the Moine Thrust, are now attributed to plate-tectonic collisions that happened almost half a billion years ago. To appreciate the classic geology of the Highlands, the basic geological field relationships remain fundamental, so this book has been written with field evidence very much to the fore. In any story, the events that took place and the order in which they did so are critical. In the story of how the Highlands were built (and then eroded), the nature and relative timing of sedimentation, metamorphism, tectonism (folding and faulting), igneous activity and erosion are all revealed by the rocks. The evidence may come from field relationships, such as those that are pictured throughout this book, or from any one of a number of radiometric dating methods requiring high-precision laboratory instrumentation. Among the latter,

7 Introduction 7 post-devonian Devonian sediments and lavas igneous rocks emplaced during Caledonian Orogeny Shetland rocks deformed and metamorphosed during Caledonian Orogeny Ordovician and Silurian sedimentary rocks Precambrian Basement fault H e thrust 100 km b r i s d e NORTH-WEST HIGHLANDS Moine Thrust Orkney Great Glen Fault r O u t e GRAMPIAN HIGHLANDS (or CENTRAL HIGHLANDS) Inverness Aberdeen Stonehaven Strontian Pitlochry Glen Clova MIDLAND VALLEY Glasgow Edinburgh Siccar Point SOUTHERN UPLANDS Highland Boundary Fault Southern Uplands Fault Figure 1.1 The physical geography and geology of Scotland are dominated by the products of mountain-building events, known collectively as the Caledonian Orogeny, that took place during the Palaeozoic. Some important localities in the historical development of the geological sciences (Glen Clova, Siccar Point and Strontian) are shown in italics.

8 8 Mountain Building in Scotland incidentally, is the rubidium strontium method. This relies on the radiogenic isotope of strontium, an element whose name derives from the village of Strontian in Argyllshire (Figure 1.1), where the element was first discovered in the mineral strontianite. 1.2 Recognizing ancient mountains The great modern mountain ranges of the world, such as the Himalaya, have been built over millions of years, and act as natural laboratories where Earth scientists can study the complex interplay of processes that are active during mountain building. The formation of these great mountains ranges, a process called orogeny, is inextricably linked to the forces generated by the collision of lithospheric plates at destructive plate margins. In these collision zones the crust is thickened by deformation, folding and faulting, and/or by the addition of large quantities of magma. A major consequence of tectonic crustal thickening is that rocks that were once at the surface are buried to great depths and undergo substantial modification during deformation and metamorphism (Figure 1.2). Figure 1.2 Highly deformed and metamorphosed igneous rocks from the Borgie inlier, north coast of Scotland. The uplift of deeply buried rocks is achieved either by erosion or by a combination of erosion and tectonic movements such as faulting, and occurs in response to the isostatic readjustment of the overthickened crust this process is called exhumation. Eventually, over millions of years, exhumation reveals the deeper levels, or roots, of the mountain belt. It follows, therefore, that the existence of ancient mountains can be recognized by the identification of zones

9 Introduction 9 of highly deformed and metamorphosed rocks these zones are called orogens or orogenic belts. The study of these ancient mountain roots provides us with a record of the processes that were active in the deep crust during collision. 1.3 Orogeny through geological time Geological time: a brief note Geological time can be divided into Eons, Eras and Periods, with further subdivisions into Epochs and Ages (Figure 1.3). The order of rock units determined from the principles of superposition and faunal succession produces the lithostratigraphic column, which is based simply on the relative ages of rocks, e.g. Llandovery is younger than Ashgill but older than Wenlock. A second aspect of the stratigraphic column relates to the chronostratigraphic dating of rock units, which allows geologists to apply absolute ages to rock successions, e.g. the Llandovery ranges from Ma. Eon PHANEROZOIC CRYPTOZOIC or PRECAMBRIAN Era PALAEOZOIC PROTEROZOIC ARCHAEAN HADEAN Period Permian Carboniferous Devonian Silurian Ordovician Cambrian Neoproterozoic Mesoproterozoic Palaeoproterozoic Late Archaean Middle Archaean Early Archaean Epoch Late Early Silesian Dinantian Late Mid Early Late Early Late Mid Early Late Mid Early Age Zechstein Rotliegendes Stephanian Westphalian Namurian Viséan Tournaisian Famennian Frasnian Givetian Eifelian Emsian Pragian Lochkovian Pridoli Ludlow Wenlock Llandovery Ashgill Caradoc Llanvirn Arenig Tremadoc Ma Figure 1.3 Part of the geological timescale, showing chronostratigraphic subdivisions. Note that formally defined divisions start with an upper case letter (e.g. Mid-Ordovician), whereas informally defined divisions (e.g. mid-silurian) start with a lower case letter. Accordingly, Early Devonian has a specific meaning or definition, whereas early Devonian is less specific.

10 10 Mountain Building in Scotland Disentangling the continents The dating of crystalline rocks using radiogenic isotopes has become a prerequisite to understanding and unravelling regions of complexly deformed and metamorphosed rocks. For several decades, radiogenic isotope systems have been used to date events such as the crystallization of metamorphic and igneous rocks. With the advent of new analytical methods it is now possible to date the crystallization of individual minerals (or even parts of minerals) such as zircon or garnet, to a precision of one or two million years, even in the oldest Precambrian rocks. Our knowledge of continental geology owes much to these methods. For instance, we now know that the succession of collisions or orogenies that have built the present-day continents occurred over long expanses of geological time. Orogenic belts can be made up of several displaced crustal fragments, called terranes, which may have travelled thousands of kilometres across the Earth s surface before colliding with and accreting to an existing continental margin. These terranes can be oceanic crust, island arcs, or pieces of continental crust carried by subducting plates and plastered to continental margins when the plates collided. The collision scar or suture zone may preserve relicts of the oceanic crust that once separated the crustal fragments these relicts are called ophiolites. The rocks that preserve a record of the oldest or most ancient orogenic episodes usually form the core or interior parts of continents these are known as shield areas or cratons. These regions are often surrounded by a series of younger, more recently active mountain belts, which form long, relatively narrow topographic features such as the Alpine Himalayan chain of southern Europe and Asia. As successively younger orogenic belts develop around the margins of the continents, the older orogenic belts that are exposed to these events become incorporated within the younger orogenies, and as a result are progressively reworked. During this process of reworking, the record of the earlier periods of orogeny becomes progressively overprinted by younger events, and so an incomplete record of orogeny is preserved. Several factors may lead to problems in reconstructing the history of ancient orogens. Remnants of older orogens are often obscured and lie beneath younger sedimentary rocks. The older rocks may have complex histories resulting from multiple episodes of deformation, metamorphism and magmatism, which in many cases cannot be resolved even by conventional isotopic methods. The diachronous nature of the successive collisions and accretion of crustal fragments to continental margins during orogeny causes problems of correlation along the belt. In addition, details of early collisions are often obscured by later accretion events. For example, plate boundary histories are difficult to unravel if large-scale strike slip fault systems have been active at different times. 1.4 The collage of ancient orogenic belts in the North Atlantic region The geology of the Scottish Highlands is dominated by the effects of the last great mountain building event to have affected the region the Caledonian Orogeny. However, the Highlands represents only a small fragment of an orogenic belt that was once of much greater extent. Fragments of this early Palaeozoic orogenic belt are scattered across the North Atlantic region as a result

11 Introduction 11 post-caledonian orogens (late Palaeozoic and later) Caledonian Orogen (early mid-palaeozoic) Late Archaean to Proterozoic Archaean cratons main sutures orogen boundaries Caledonian Belt B A L T I C A Tornquist Belt L A U R E N T I A EASTERN AVALONIA ARMORICA Iapetus Suture Appalachian Belt W E S T E R N I A A V A L O N GONDW ANA IBERIA Figure 1.4 Map showing the distribution of ancient continental fragments around Britain and Ireland prior to the opening of the present-day Atlantic Ocean. The age assigned to different continental fragments is that of the most dominant period of orogeny. The younger orogenic belts are built on reworked older orogenic belts. Light shades indicate areas where the geological reconstruction is inferred. of the opening of the present-day Atlantic Ocean. A reconstruction of the position of the continents prior to this opening clearly demonstrates the extent of the Caledonian Orogenic Belt (Figure 1.4). Examination of Figure 1.4 reveals that the Caledonian Orogenic Belt (the Caledonides) has three distinct arms: a northern Caledonian Belt, a western Appalachian Belt and an eastern Tornquist Belt. These belts or branches separate parts of three major palaeocontinents that preserve records of much older orogenic episodes, e.g. the palaeocontinent of Laurentia is composed of rock units that were subjected to major periods of orogeny in both the Archaean and the Proterozoic. The three palaeocontinents of Laurentia (North America and Greenland), Baltica (Scandinavia and the Baltic) and Avalonia (southern Ireland and Britain) drifted together in the early Palaeozoic, colliding to form the Caledonian Orogenic Belt. Britain and Ireland hold a special position in this orogenic system as they straddle the Caledonian Orogen, adjacent to the three palaeocontinents.

12 12 Mountain Building in Scotland 1.5 What caused the Caledonian Orogeny? The tectonics that led to the construction of the Caledonian Orogenic Belt were associated with the closure of a major ocean called Iapetus. The Caledonian Orogeny was not a simple continent continent collision, it encompassed a series of more localized arc arc, arc continent and continent continent collisions. The sequence of events that led to the collision of Laurentia, Baltica and Avalonia, and to the formation of the Caledonian Orogenic Belt, can be traced as far back in time as c. 600 Ma, to the break-up of a major palaeocontinental landmass referred to as the Vendian Supercontinent, named after the Vendian Period (c Ma). This sequence of events is illustrated on Figure 1.5 and can be summarized as follows: The initial break-up of the Vendian Supercontinent began c Ma ago with Baltica rifting from eastern Laurentia along the Greenland margin, and Gondwana rifting from North America, thus forming the Iapetus Ocean (Figures 1.5a,b). The initial stages of ocean destruction began in Cambrian to Early Ordovician times with the formation of subduction zones and the localized collision of volcanic arcs along the margins of the continents that surrounded Iapetus (Figures 1.5c,d). The phase of arc continent collisions along the southern margin of Laurentia is referred to as the Grampian orogenic phase, evidence for which can be seen in the rocks of Scotland and Ireland. Also at this time the continent of Gondwana began to break up, and continental fragments, including Avalonia, which incorporated southern Britain, rifted and migrated northwards, narrowing the intervening Iapetus Ocean and opening another, the Rheic Ocean, in its wake (Figure 1.5d). The initial collision of Avalonian fragments with the Laurentian margin (Figure 1.5e) occurred between c. 470 Ma and c. 440 Ma. The final closure of Iapetus was in the Silurian (c. 425 Ma), with Baltica and parts of Avalonia converging obliquely with the Laurentian margin (Figure 1.5f), resulting in the Scandian orogenic phase. At this time, significant strike slip displacements associated with oblique collision disrupted the Laurentian margin and the Iapetus Suture Zone. 1.6 The tectonic map of Britain and Ireland The Caledonian Orogeny was primarily responsible for the consolidation of the British Isles into its present pattern of fault-bounded crustal blocks. These crustal blocks are terranes that represent displaced fragments of continents, volcanic arcs or ocean basins accreted to the continental margins by combinations of subduction, collision and strike slip displacement. Each of these terranes has its own individual and distinct history. The terranes and major terrane-bounding faults of Britain and Ireland are illustrated in Figure 1.6. These terranes are subdivided into three main groups: Laurentian Terranes: those to the north of the Highland Boundary Fault Fair Head Clew Bay Line, which have evolved as part of Laurentia (Hebridean, Northern Highlands and Central Highlands Terranes). Gondwanan Terranes: those to the south of the Solway Line, which are thought to have evolved as part of Gondwana (Leinster Lakesman, Monian, Welsh Basin and Midland Platform Terranes). Intermediate Accreted Terranes: an intervening zone consisting of slivers of continental margin, island arc and oceanic rocks (Midland Valley and Southern Uplands Terranes) that form a complex suture zone separating Laurentia and the Gondwanan Terranes.

13 Introduction 13 (a) c. 580 Ma Vendian supercontinent SIBERIA (b) Late Neoproterozoic c. 550 Ma SIBERIA 30 S 60 S BALTICA L A U R E N T I A N South Pole S GONDWANA L A U R E N T I A N BALTICA 60 S South Pole S GONDWANA 30 S N Pacific Ocean (c) Early Ordovician c. 490 Ma L A U R E N T I A A r c Tornquist Sea Iapetus Ocean G r a m p i a n SIBERIA S BALTICA South Pole 60 S GONDWANA 30 S Equator N S main regions of rifting and ocean opening major subduction zones positions of northern and southern Britain and Ireland Equator N S Iapetus Ocean (d) Mid-Ordovician c. 470 Ma L A U R E N T I A SIBERIA Tornquist Sea AVALONIA Rheic Ocean BALTICA South Pole 60 S GONDWANA 30 S Equator (e) Early Silurian c. 440 Ma (f) Mid-Silurian c. 425 Ma SIBERIA L A U R E N T I A N S AVALONIA ARMORICA South Pole BALTICA Rheic Ocean Theic Ocean 60 S 30 S Equator 30 N Equator BALTICA 30 S L A U R E N T I A 60 N SIBERIA Rheic Ocean ARMORICA GONDWANA GONDWANA 60 S South Pole Figure 1.5 Global palaeocontinental reconstructions for the late Neoproterozoic and Palaeozoic: (a) c. 580 Ma; (b) c. 550 Ma; (c) c. 490 Ma; (d) c. 470 Ma; (e) c. 440 Ma; (f) c. 425 Ma.

14 14 Mountain Building in Scotland terrane boundaries are delineated according to date of last major displacement 100 km Devonian Ordovician or Silurian Cambrian or earlier Fair Head Clew Bay Line HEBRIDEAN NORTHERN HIGHLANDS Moine Thrust CENTRAL HIGHLANDS (GRAMPIAN) Southern Uplands Fault Great Glen Fault Highland Boundary Fault MIDLAND VALLEY SOUTHERN UPLANDS Solway Line =lapetus Suture Zone LEINSTER LAKESMAN LAURENTIAN TERRANES INTERMEDIATE ACCRETED TERRANES GONDWANAN TERRANES MONIAN WELSH BASIN MIDLAND PLATFORM Variscan Front? LIZARD Figure 1.6 Simplified Palaeozoic terrane map of Britain and Ireland. 1.7 The scope of this book The Scottish Highlands contains some of the most extensively studied areas of geology in the world. Given the fact that the region is of relatively small size, it is perhaps surprising that the rocks of the Scottish Highlands preserve a record of at least six periods of mountain building, which occurred over considerable geological time. In this book we will examine the nature of the evidence and the methods employed by geologists to unravel the complex history of this particular region. As mentioned in Section 1.4, the Caledonian Orogeny was the last major mountain-building event to have left an imprint on this region. In Sections 2 and 3 we will see glimpses of several older orogenic episodes from the Archaean and Proterozoic Eras. These serve to outline the basic concepts and principles of orogenesis, from which a more detailed study of the Caledonian Orogeny can be undertaken. Sections 4 to 9 focus on unravelling what happened during and after the Caledonian Orogeny.